Power supply manufacturers must meet government (e.g., Federal Communications Commission) and customer electromagnetic interference (EMI) emission requirements when selling switch mode power supplies. A switch mode power supply may use pulse width modulation (PWM) to control a switch, e.g., power transistor, power field effect transistor, etc., that may charge an inductor to a desired current. The longer the PWM signal is on, the longer the inductor is charging through the control switch and the more current that may be supplied from the switch mode power supply.
In the larger capacity switch mode power supplies, the power transistors generate electromagnetic interference (EMI). These larger capacity switch mode power supplies must be tested for and pass stringent EMI tests that limit the amount of radio frequency energy at a particular frequency for a certain length of time. If the switch mode power supplies cannot meet these EMI test requirements, they cannot be certified and/or type accepted by the appropriate testing organization and/or government agency.